1. Field of the Invention
This invention relates to the field of nondestructive evaluation and testing, and in particular to the nondestructive testing of composite blades for wind turbine electricity generators.
2. Description of the Related Technology
Electric power generation from wind has been growing at a rate of 20 to 30% per year, and it is one of the fastest growing market segments in the power industry. According to American Wind Energy Association statistics for 2009, over 10,000 megawatts (MW) of wind power was installed in the United States. The current U.S. wind power capacity is over 35,000 MW with wind providing 39% of all new generating capacity in 2009. The development of U.S. wind energy resources is envisioned as one of the key enablers in meeting future renewable energy generation goals. Presently, the primary focus has been on development, installation, and start-up of wind energy resources. The future success of wind power will depend on consistent, reliable operation, and cost-effective maintenance of the wind assets.
The current generation of megawatt sized wind turbines are up scaled versions of earlier large kilowatt sized designs. Wind turbine towers are taller and the rotor diameters larger in order to capture as much wind energy possible; and the upsizing trend will continue. Increasing wind turbine size and output has resulted in issues with wind turbine reliability. Wind turbine components are failing prematurely which results in increased maintenance costs and downtime; and loss of revenue. Periodic maintenance inspections are performed on major wind turbine components; however, these inspections do not always identify the conditions leading to failure. Three wind turbine components with the highest incidence of failure are the gearboxes, generators, and blades. These components also have the highest cost consequences from a failure due to high costs for replacement parts, high cost to implement remedial actions (crane costs), and lost revenue caused by extended and unplanned down time.
Currently, there are no regulations, codes, or standards to regulate the operation and maintenance of wind turbines or their components. Wind asset owner/operators periodically inspect major components in accordance with the wind turbine manufacturer's or internal recommendations. Due to their size and access, periodic inspection of wind turbine blades is more difficult. Typically, the blades are visually inspected by maintenance personnel who are suspended by ropes, or use special platforms to traverse up or down the blade. These personnel perform visual examinations of the exterior surfaces for detection of flaws and damage that could be detrimental to the operability of the blade. In some cases, personnel may crawl through the interior of blade by accessing the rotor hub area to visually examine the internal structural of the blade. Recently, some jurisdictions have considered these up tower internal blade examinations to be “confined space” entries and imposed restrictions on internal blade examinations.
While the performance of periodic visual examinations of wind turbine blades is a good practice, most visual examinations are limited to the extent of flaws or damage detected since it cannot detect flaws or determine the condition of the structure underneath the blade surfaces. In addition, the quality of the examination can be highly dependent on the experience of the examination personnel; access, distance, and angle to the examination surface, and available lighting. In general, visual examination is a viable technique for determining the general condition of the blade, but may not be adequate to assess the overall structural integrity of the blade.
Wind turbine blades are aerodynamically designed structures that are constructed primarily of fiberglass or carbon fiber reinforced composite materials. The manufacture of these large composite structures is a difficult process that is normally performed with skilled manual labor. Wind turbine blades are typically constructed in two halves. Each blade half consists of an exterior skin constructed of multiple layers of fiberglass or carbon fiber material bonded to structural elements. These structural elements, such as spars and webs, add strength and rigidity to the blade to transfer the wind load back through the rotor hub, to spin the turbine/generator. The two blade halves with structural elements are assembled and bonded together using an epoxy type resin to form a bond between the mating surfaces of the two halves.
During the blade manufacturing process, fabrication flaws may occur due to manufacturing process and tolerance anomalies or problems during the resin addition and bonding process. The fabrication flaws include:                Delamination between layers of composite material,        Wrinkles, or waviness between layers of composites material,        Lack of bond or de-bonds between bond lines of structural elements and leading and trailing edges of blade.        
Due to the complexities in the blade manufacturing process, most blades contain some type of fabrication flaw before they enter into service. Many fabrication flaws are not visible to the surface, and visual examination is not effective method to detect and assess the effect of the fabrication flaws on the blade during in-service conditions.
Visual examinations will provide information on the general condition of the visually accessible surfaces of a recently manufactured or an in-service wind turbine blade. However, the performance of visual examinations may be variable and subjective; and the information may not be sufficient to adequately assess the overall integrity of the blade. A small flaw visually observed on the blade surface may not be indicative of extensive damage in the structure underneath the blade skin. Thus, in order to perform a more comprehensive assessment of a blade's structural integrity, the examination should be capable of detecting flaws at or below the surface, or through the volume of blade.
Other NDE methods are available and used for wind turbine blade inspections that may provide enhanced information about the structural integrity of a blade. These methods consist of two general categories: 1) surface or near-surface examination techniques for detection of flaws at/or near the component surface; or 2) volumetric examination techniques for detection of flaws within the volume of the component. Surface examination techniques include:                penetrant testing (PT),        eddy current testing (ET),        thermal imaging, and        optical imaging techniques such as laser shearography, and digital image correlation (DIC)Volumetric Examination Techniques Include:        ultrasonic examination (UT), includes conventional and phased array UT, and guided wave UT        bond testing—a form of UT        radiography (RT)Other NDE Techniques Include:        tap-testing—an audio technique to detect areas of lack of bond in composite materials        acoustic emission—use of piezo-electric sensors to detect changes in component strain        
The prime factor for performing comprehensive wind turbine blade inspections is the ability to examine large surface areas with little or no contact with the component surface. All of the above NDE methods have been used to perform examinations on wind turbine blades, and each will provide examination information within the scope of the technique's capability. There are advantages and disadvantages in applying any of these techniques to examine a wind turbine. For example, ultrasonic examination is very good for detecting subsurface lack of bond, and other structural bonding anomalies. However, the ultrasonic transducer/probe must in contact with the part and only a small area/volume underneath the probe is examined. Acoustic emission (AE) is used during blade testing to detect the stress waves (audio) that are released in a material when it is subject to an external load/stress. Multiple AE sensors must be attached to the component and the component loaded in order to detect the presence of potential flaw locations. While AE is practical for monitoring blade testing activities, it would be difficult to implement for post-fabrication and in-service examinations of blades.
Radiography requires access to both sides of a component to place the source and film, which is not always practical. For thermal imaging, the detection capability is related to the thermal depth of penetration which may be limited due to the ability to heat-up of the large examination area.
Currently, visual examination is predominately used to determine the condition of wind turbine blades. No other NDE method has been developed that will provide a fast assessment of the overall structural integrity of the blade. With surface areas from 65 to 278 sq. meters (700 to 3,000 sq. ft.), or more, wind turbine blade inspection pose formidable challenges for both manufacturing and field inspection. In order to develop a better alternative to visual examination, an NDE technique should be able to examine these large surface areas, with little or no contact with the component surface.
Laser shearography NDE using current portable thermal, vacuum and acoustic energy stress techniques are well known, such as is disclosed in U.S. Patents, Newman et al, U.S. Pat. Nos. 5,146,289; 5,257,088; and 6,717,681, the entire disclosures of which are incorporated by reference as if set forth fully herein. However, such techniques require the shearography camera be in relatively close proximity, typically from 10 inches to 10 feet, to the area on the test part being inspected. This requirement is caused by environmental degradation of the image due to test part motion, air currents with temperature and density gradients that refract the laser light used to illuminate the test object surface or light that reflects from the surface to the camera. An additional limitation in current shearography systems is the image degradation due to relative movement between the camera and the test object. Many portable shearography systems require physical placement of the inspection devices on the blades requiring rope, crane or sky lift platforms to gain access to blade areas requiring inspection.